1. Field of the Invention
This invention relates to apparatus for planar flow casting of metal ribbon, particularly ribbon of amorphous metal alloys.
2. Description of the Prior Art
For purposes of the present invention, a ribbon is a slender body of substantially rectangular cross section whose transverse dimensions are much smaller than its length.
Apparatus for preparing continuous metal ribbon from the melt by the planar flow casting method is disclosed, inter alia, in U.S. Pat. No. 4,142,571, issued Mar. 6, 1979 to Narasimhan. The disclosure of this patent is incorporated herein by reference. It provides apparatus which comprises a movable chill body, a slotted nozzle in communication with a reservoir for holding molten metal and means for effecting expulsion of the molten from the reservoir through the nozzle onto the moving chill surface.
The slotted nozzle is located in close proximity to the chill surface. Its slot is oriented perpendicular to the direction of movement of the chill surface and is defined by a pair of generally parallel lips, a first lip and a second lip numbered in the direction of movement of the chill surface. The slot has a width, measured in the direction of movement of the chill surface, of about 0.2 to 1 mm. There is no limitation on the length of the slot (measured perpendicular to the direction of movement of the chill surface) other than the practical consideration that the slot should not be longer than the width of the chill surface. The length of the slot determines the width of the strip or sheet being cast.
The first lip has a width (measured in the direction of movement of the chill surface) at least equal to the width of the slot. The second lip has a width of about 1.5 to 3 times the width of the slot. The gap between the lips and the chill surface is at least about 0.1 times the width of the slot, but may be large enough to equal the width of the slot.
As the molten metal is forced onto the chill surface, a molten puddle is formed. The puddle extends a short distance upstream, forming a meniscus extending between the chill surface and the first lip of the nozzle. It has been discovered that a unique equilibrium position for the meniscus does not exist when the first lip consists of a single plane surface parallel to the chill surface as is disclosed in the prior art. Variations in the meniscus position (i.e., distance from the slot) give rise to undesirable variations in the dimensions of the cast product.
Ribbon prepared by the casting methods of the prior art often suffer from wavy and uneven edges caused by turbulent air movement over the surface and sides of the molten metal. For chill block melt-spinning, the parameters of this problem were recognized in U.S. Pat. No. 4,144,926, issued on Mar. 20, 1979, to H. H. Liebermann. It is not clear that the Liebermann's conclusions are applicable to planar flow casting. In any case, no nozzle configuration has been suggested that would provide the uniform ribbon surface and even edges that are desirable.
Air flow in the vicinity of the molten metal puddle can also cause defects in the bottom surface and in the body of the ribbon. These are created when air is trapped between the molten metal puddle and the chill surface. Trapped air can caused uneven bottom surface and/or voids in the body of the ribbon. Also, to the extent that entrapped air reduces the area of molten metal contact with the chill surface, quench rate is reduced, which is particularly undesirable when amorphous or metastable ribbon is being cast.
Although these problems can be mitigated by casting under a vacuum (see e.g., U.S. Pat. No. 4,154,283, issued May 15, 1979, to R. Ray et al.), casting in air is more convenient.